The invention relates to a linear position measuring system and to a method for determining the absolute position of a carriage along a slide rail.
For example, systems for determining the position of a carriage are used in combination with guide systems, e.g., linear guides, which encompass a first body and a second body guided on the first body that can move relative to the first body, and here have the job of making it possible to determine the position of the second body relative to the first body. To this end, for example, a measuring scale of the respective device for determining a position can be fixed in place relative to the first body, for example, and a respective scanner can be fixed in place relative to the second body.
For example, there are linear position measuring systems known in the art for determining an absolute position that encompass the measuring scale marked with measuring points and a scanner that can move relative to the measuring scale for scanning the respective measuring points. For example, these measuring points consist of one or more acquirable markings to identify a position. The markings can be acquired optically or magnetically, for example.
In optical scanning, the scanner encompasses a sensor for acquiring an image of the measuring points and providing signals making it possible to determine the position of the scanner relative to the measuring scale. In magnetic scanning, the scanner encompasses a magnetic field sensor for acquiring a magnetic field progression of individual permanent magnets, which in this case make up the measuring points of the measuring scale.
Depending on the respective measuring scale (optical/magnetic), these types of systems can be used, for example, to measure a relative change in the position of the scanner in relation to an initial position, or to record an absolute position of the scanner.
To reach a point where these types of systems become able to measure relative changes in position of the scanner in relation to the measuring scale, for example, the respective measuring scale can be designed as an incremental scale, and consequently acquire a sequence of several identical, periodically arranged markings spaced apart at equal distances along a prescribed line or measuring scale. For example, to enable the optical scanning of such an incremental measuring scale, the scanner can project an optical image of the respective markings onto a sensor in the form of a photoelectric detector. To measure relative changes in position of the scanner in relation to the measuring scale, the scanner is moved along the track of markings. Moving the scanner here causes a signal to change periodically, for example providing information about how many markings the scanner passed by.
In addition, signals recorded for various positions of the scanner along the track of markings can be interpolated, making it possible to determine the position of the scanner relative to the markings to within an inaccuracy of less than the distance between adjacent markings.
In sum, the respective change in the relative position of the scanner can be determined by scanning the measuring points of an incremental measuring scale. So-called incremental position encoders are used for this purpose, which have a comparatively simple design. They offer relatively high resolutions.
Aside from acquiring the relative movement between the slide rail and track carriage or scanner, the system can also be designed to determine an absolute position of the scanner in relation to a reference scale. The respective absolute position of the scanner can here be determined at any location along the slide rail by measuring a change in the relative position of the scanner in relation to a specific reference point. The reference point must here be scanned in an especially reliable manner, since any misinterpretation would lead to completely erroneous information about the position of the carriage in relation to the slide rail.
To this end, the reference scale can exhibit one or more reference points along a predetermined line, which each specify a certain absolute position. In order to determine the position, the aforementioned scanner can be moved along the predetermined line, so as to optically or magnetically scan the respective reference points by means of the scanner.
In sum, these so-called absolute encoders always transmit the position-related information in its entirety, which makes them very well suited for determining and controlling position. The conventional approach involves reading out a piece of binary information, wherein a corresponding optical or magnetic scan is needed for each binary digit. All of these scans must be adjusted relative to each other so that no read error can arise under any circumstances. As the requirements placed on scanning accuracy become ever more stringent, the computation time and power consumption for this purpose increase.
In the past, the reference points were evaluated using only a simple threshold circuit. Exceeding the threshold causes the function to be digitized. The advantage to this is that plausibility examinations can be performed. The reference signal is here generated by a half bridge. In terms of practical implementation, this half bridge can be realized in the form of the north/south pole of a permanent magnet. An alternating signal results from scanning the magnetic field of a permanent magnet during the relative movement of a scanner along the permanent magnet. For example, the threshold can be set as a function of the desired sensitivity and the amplitude of the alternating signal, or the distance between the peak values (peak-peak). The distance between the peak values can here be influenced by the magnetization width. The position of the peak values is here independent of the measuring signal strength.
One problem in this approach toward determining the position by scanning the reference point is that the reference pulse width can be varied in relation to the signal amplitude. In the event of a super-elevated signal amplitude, this can result in a reference pulse that is too wide for reliable evaluation. By contrast, if the signal amplitude is too low, reference pulse detection may be mistakenly omitted altogether.
Another problem lies in the occurrence of overshoots. In practice, an output variable will not reach the desired value after a sudden change in an input variable, e.g., the magnetic field of the permanent magnets, but rather will overshoot the set value, and only adjust or settle into the desired value thereafter. In cases where the reference point is acquired by scanning a magnetic field progression, this phenomenon arises primarily when the reference sensor is in a highly saturated region. As a result, magnetic field lines can be acquired that emerge in the near field of the reference pulse in the opposite direction. This distorts the result of reference point scanning.
Another problem associated with acquiring the reference point by scanning a magnetic field progression is that mechanical tools with a residual magnetization can apply magnetic poles to the reference track after the fact. Since these imposed interferences result in magnetic poles having a respective width longer than the width of the permanent magnets of the reference track, this leads to a lower amplitude. In addition, the distance between the peak values is reduced. The measuring result of the reference point scan as a whole are greatly distorted as a result.